The stress test shows if your heart receives enough blood from its own arteries to work harder, safely. Taking the stress test also helps your doctor know what type of exercise and how much is right for you.
A comparison of SPECT and PET images- Source image-Wikipedia
A nuclear stress test combines the treadmill portion with pictures (images) recorded on computerized gamma cameras. For the pictures, the patient receives a small dose of a radioactive substance (isotope), which the cameras can see. The radioactive substance can be thallium-201 or technetium-99msestamibi (Mibi).
You can find further information here: The American Heart Association Website